THE STORY OF NINETEENTH-CENTURY SCIENCE 



earlier. It was suspected by Stokes and by Fox Talbot 

 in England, but iirst brought to demonstration by Kirch- 

 hoff and Bunsen, that these lines, which were known to 

 occupy definite positions in the spectrum, are really in- 

 dicative of particular elementary substances. By means 

 of the spectroscope, which is essentially a magnifying 

 lens attached to a prism of glass, it is possible to locate 

 the lines with great accuracy, and it was soon shown 

 that here was a new means of chemical analysis of the 

 most exquisite delicacy. It was found, for example, 

 that the spectroscope could detect the presence of a 

 quantity of sodium so infinitesimal as the one two- 

 hundred-thousandth of a grain. But what was even more 

 important, the spectroscope put no limit upon the dis- 

 tance of location of the substance it tested, provided 

 only that sufficfent light came from it. The experi- 

 ments it recorded might be performed in the sun, or in 

 the most distant stars or nebulae ; indeed, one of the 

 earliest feats of the instrument was to wrench from the 

 sun the secret of his chemical constitution. 



To render the utility of the spectroscope complete, 

 however, it was necessary to link with it another new 

 chemical agency, namely, photography. This now fa- 

 miliar process is based on the property of light to de- 

 compose certain unstable compounds of silver, and thus 

 alter their chemical composition. We have seen that 

 Daw and Wedgwood barely escaped the discovery of 

 the value of the photographic method. Their successors 

 quite overlooked it until about 1826, when Louis J. M. 

 Daguerre, the French chemist, took the matter in hand, 

 and after many years of experimentation brought it to 

 relative perfection in 1839, in which year the famous 

 daguerreotype first brought the matter to popular at- 



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